Methylene chloride has long been used in paint strippers because it is inexpensive and highly effective. Because of concerns about the toxicity of methylene chloride, less-toxic, non-halogenated alternatives such as N-methyl-2-pyrrolidone, dibasic esters, and other solvents are replacing methylene chloride in paint strippers.
N-Methyl-2-pyrrolidone has been used in numerous methylene chloride-free formulations, including those described in U.S. Pat. Nos. 4,664,721, 4,836,950, 4,749,510, 4,780,235, 4,120,810, 4,732,695, 5,011,621, and 5,015,410. These formulations include substantial amounts of aliphatic or aromatic hydrocarbons, dibasic esters, or oils. Unfortunately, water rinsing of these coatings is not feasible because of the low water solubility of hydrocarbons, oils, and dibasic esters.
Water-rinsable compositions are taught in U.S. Pat. No. 4,927,556, in which diesters of C.sub.4 -C.sub.6 dibasic acids are combined with water and a thickener. European Patent Application 0 389 829 teaches a water-reducible formulation that incorporates NMP, diesters of C.sub.4 -C.sub.6 dibasic acids, glycol ethers, and a thickener. U.S. Pat. No. 5,006,279 teaches a water-reducible composition that includes NMP, aromatic solvents, a glycol ether, an alkanolamine, and a thickener. International Publication No. WO 88/06640 teaches a water-soluble paint remover containing NMP, a glycol ether, and an organic acid. U.S. Pat. No. 5,024,780 teaches a graffiti remover comprising NMP, propylene carbonate, a glycol ether acetate, isocetyl alcohol, a thickener, and a surfactant.
Although these compositions are water-rinsable, they suffer from a number of drawbacks. Dibasic esters are a potential health hazard, as indicated by their low permissible exposure limit (PEL) of 10 ppm. Organic acids are corrosive. Isocetyl alcohol is expensive. Many aromatic hydrocarbons evaporate easily and aggravate lower atmosphere ozone problems. In addition, compositions containing substantial amounts of aromatic hydrocarbons or dibasic esters often phase separate during storage. Performance of many water-rinsable compositions previously developed is also somewhat poorer than desirable for many types of coatings, including cured enamels, two-part polyurethanes, and nitrocellulose lacquers.